Rock crushing plants are used for the production of aggregates. Within a rock crushing plant there are usually three stages of crushing: primary crushing, secondary crushing, and tertiary crushing. Quarry rock is fed to a primary crusher in order to reduce the size of the rock to below a given maximum size. Typically a Jaw crusher or Gyratory crusher is used in the primary crushing stage. The size of the quarry rock is reduced to 8 inches in diameter or less (minus 8 inches) by the primary crusher, and is then conveyed to a stockpile.
The stockpile generated by the primary crusher is transferred onto a conveyor by a feeder, delivered to screens for classifying the rock, and then to a secondary crusher. In ordinary plant operations, only one secondary crusher is required. The secondary rock crusher is capable of reducing the size of the rock to less than a given size normally minus 4 inches. It is not possible to control the minimum particle size that will be produced. The desired maximum diameter of the rock being crushed depends upon the intended use of the rock, whether it be for concrete aggregate, roadstone, or a finer product, such as sand. Some of the rock leaving the primary and secondary stages will be reduced in size enough so that no further crushing is required. The remainder of the rock will need to be crushed in the secondary and tertiary crushers respectively. Accordingly, the output of the primary, secondary and tertiary crushing stages go through classifying screens so that only the larger diameter rock is crushed in the secondary and tertiary crushers respectively. The smaller diameter rock that is the size of the desired product is temporarily stored and later transported out of the plant as a final product.
The output of the secondary crusher is classified to remove the dust and smaller diameter rock with screens. The larger diameter rock is conveyed to a surge pile and then fed to a tertiary crusher. Cone crushers are usually used in the tertiary stage, and for very fine tertiary crushing, Gyradisc crushers are used. The maximum size of the tertiary crusher output can be chosen by setting a desired gap dimension between the crushing surfaces of the crusher. As with the secondary crushers, the product of the tertiary crushers needs to be classified to obtain the desired final product.
In a rock crushing plant, each of the primary, secondary and tertiary stages is operated independently of the other. That is, the feed to the secondary crusher is obtained from a stockpile. Likewise, the feed to the tertiary crusher is obtained from a bin or surge pile. As a result, the focus of optimizing overall plant throughput is divided into optimizing the throughput for each of the crushing stages with priority being given to the least productive stage.
The difficulty in optimizing the efficiency of the crushers in a rock crushing plant relates to the extremely hazardous environment in which the process control equipment must operate and the constantly changing variables that must be accounted for. Sensing equipment that is intended to contact the rock, such as a level sensor or the like, must withstand occasional, severe impacts and also withstand the penetration and build-up of fine particulate matter, such as rock dust. Further, the system must be able to accommodate changes in operating parameters that are frequently changed by the operators in accordance with their needs.
The variables that are subject to change during a run include differences in hardness, size, and moisture content. For example, the feed at the beginning of a run will have a smaller average diameter than at the end of the run due to segregation of the rock in the surge pile from which the rock is fed. Also, the rock at the bottom of a pile will have a different moisture content than the rock that has been laying on the surface of the pile. Therefore, effective throughput control systems for rock crushers have been slow in development.
Modern size reduction equipment has been designed to operate more efficiently in accordance with the recognized need to increase throughput. Replacing equipment in a rock crushing plant, however, is ordinarily one of the least viable alternatives to the owner, because the equipment, such as the crushers, is so expensive. As a result, a great need has developed for process control systems that can optimize the crusher efficiency and thereby increase the crusher throughput of existing crushers. Some of the variables that affect the operation of a crusher during a given run can be assumed to be fixed to a certain extent. For example, the hardness of the rock in a given run will remain substantially the same. Other variables cannot be fixed with such certainty, however, because they change as the length of the run continues. For example, the size of the rock and its moisture content changes as the surge pile is reduced. As another example, the setting of the gap between the crushing surfaces of the crusher will widen as the surfaces wear, and the rate of wear will depend upon the hardness of the rock being crushed. Therefore, for a control system to operate a crusher efficiently, it must take into account all of the variables, and deal with them whether they are fixed or subject to change.
The most controllable and result effective variable is the feed rate of rock delivered to the crusher. For cone crushers, the feed rate should be increased until the crushing cavity is filled, but not increased so much that the rock overflows the crusher. This results in the most efficient operation of a cone crusher. When the feed rate is such that the crushing cavity is always full, then the crusher is being choke fed. To ensure that the choke fed condition is maintained, the crusher bowl can be kept full and the feed rate controlled so that no overflow condition occurs. As the crushing cavity fills, the horsepower requirement for the prime mover of the crusher increases. When the crushing cavity is completely full and the crusher is operating under a choke fed condition, the motor driving the crusher operates within a peak range, and the feed rate can thereafter be controlled by monitoring the horsepower of the motor and adjusting the feed rate accordingly. As the crushing surfaces of the crushing cavity wear, however, the horsepower decreases and a control system operated by sensing horsepower alone would increase the feed rate and eventually overflow the crusher bowl, without timely intervention by an operator. To alleviate the overflow problem, and to signal an operator to reset the crushing cavity gap, a level control device, such as a level probe, can be used to signal the control system that adding feed will cause an overflow condition. If the gap is not then reset, the crusher can continue to operate by simply increasing the feed rate when the level control device indicates that the crusher bowl is below a predetermined level, and decreasing the feed rate when the level control device indicates that the crusher bowl is filled above that level.
Control systems of the type mentioned are known. A programmable logic controller has been used to vary the feed rate to the crusher in accordance with signals received from a horsepower sensor and a level sensor so that an optimum feed rate for the present conditions is delivered to the crusher. Accordingly, the control system automatically accounts for changes in moisture content, rock size, and the wearing of crushing surfaces. This type of control system, in theory, therefore is adequate to increase crusher throughput by ensuring that the crusher cavity is always filled and therefore that the crusher is operated in a choke fed condition.
In practice, however, this type of control system is barely workable for many crushers presently operating in rock production plants. The secondary and tertiary crushers of these plants are fed from a stockpile or surge pile located a significant distance away from the mouth of the crusher. The rock must travel a long way from a feeder at a surge pile along a series of conveyor belts to the mouth of the crusher. As a result, when a control system commands the feeder to increase or decrease the feed rate in accordance with a sensed condition of the prime mover, or level sensor, the response time is too long and the unwanted condition occurs anyway. Locating the feeder closer to the crusher in order to alleviate this problem is impractical, if not impossible, because of the fixed constraints of the overall plant design. Also, the conveyors for transporting the rock cannot be eliminated or shortened, because the maximum slope of the conveyors cannot be increased. As a result, many of the secondary and tertiary cushers operating in rock production plants are being operated at less than 50 percent of their maximum efficiency, which represents the fact that most crushers in present use are not being choke fed.